(1) Field of the Invention
The present invention relates generally to a process for extracting and refining soybean oil and to the resultant product, and in particular to a soybean oil process that does not require caustic refining to remove free fatty acids.
(2) Description of the Prior Art
Soybean oil production involves several steps that are necessary to render the soybean oil suitable for human consumption. These production steps may be broadly characterized as 1) soybean preparation, 2) oil extraction, and 3) oil refining. Soybean preparation generally includes the steps of cleaning, drying, cracking, and dehulling. These steps are well known in the prior art.
Oil extraction is for the purpose of separating the oil from the remainder of the soybean, known as soybean meal. In a prior art process known as expelling, the oil is extracted by passing the dehulled beans through a screw press to crush the beans and separate the oil from the meal. This process has rarely been used to process soybeans due to the fact that a large percentage, up to 25%, of the soybean oil is left in the meal.
The great majority of commercial soybean extraction processes use a solvent to separate the oil from the meal. In the solvent extraction process, the beans are flaked to provide a large surface area. A solvent, commonly hexane, is then pumped through the soybean flakes, dissolving the oil in the hexane. The hexane is then separated from the oil and recycled.
The crude oil resulting from the extraction process must then be subjected to additional treatments, collectively called xe2x80x9crefiningxe2x80x9d, to remove various materials in order for the oil to be suitable for consumption. These materials include hydratable and non-hydratable phospholipids, free fatty acids, and various color and flavor components.
Crude soybean oil contains phosphorous compounds called hydratable phospholipids, and small amounts of calcium and magnesium that complex with a portion of the phospholipids to form non-hydratable phospholipids. Hydratable phospholipids are normally removed by a process known as xe2x80x9cdegummingxe2x80x9d, in which the oil is agitated or otherwise intimately combined with water to precipitate gums from the oil. The gums are then removed by centrifugation.
These precipitated gums can be used as a feed additive, or evaporated to remove moisture, the end product is called lecithin. Lecithin has various end uses such as food emulsifier. The degummed oil is dried under vacuum to remove any water. Removal of non-hydratable phospholipids is considerably more difficult and expensive, requiring further chemical treatment, typically chemical refining, to break the chemical bonds between the calcium or magnesium ions and the phospholipids, followed with extensive bleaching of the oil.
In most processes, free fatty acids are then removed from the oil by a process known as caustic refining, also called chemical or alkali refining, in which the oil is mixed with a caustic material, such as sodium or potassium hydroxide, which undergoes a saponification reaction with the acids, forming soaps that are then removed by centrifugation. In this case, the non-hydratable phosphotide are removed along with the free fatty acids. Chemical refining soybean oil is an expensive process, requiring a large investment in capital equipment. In addition, a significant quantity of the oil is captured by the soaps, adversely affecting oil yield. Also, the caustic refining process produces soapstock, which has no commercial value, and it is difficult to dispose of without environmental problems. Therefore, attempts have been made to develop more economically viable processes for removal of free fatty acids.
Free fatty acid removal by a process known as physical refining has been used for oils that are low in non-hydratable phospholipids, such as lauric oils, particularly palm oil. In physical refining, the oil is vacuum distilled at high temperatures, e.g., from about 450xc2x0 F. to about 500xc2x0 F., to separate more volatile components from the oil. This process is used to remove various flavor components, and will also remove free fatty acids. However, the process has not been viable for removing free fatty acids from oils such as soybean oil, which contains higher levels, i.e., more than 20 ppm based of phosphorous content, of non-hydratable phospholipids. The high temperatures required for physical refining tend to break down the non-hydratable phospholipids that are present in the soybean oil, producing chemical compounds that cause an unacceptable flavor and color.
As noted previously, the amount of non-hydratable phospholipids can be reduced by additional treatment steps involving chemical treatment, followed by extensive bleaching. However, these steps add substantially to the cost of production. For example, formerly described processes for physical refining of soybean oil require the removal of non-hydratable phospholipids with the use of at least 0.60% silica and 1.5% clay based on the weight of the oil. Since the silica and clays cannot be reused, these quantities add considerably to the cost of production. Moreover, a significant quantity of oil is lost in these high dosages of silica and clay. These high costs make these processes unattractive compared to chemical refining.
Conventional refining processes also involve some bleaching of the soybean oil to remove color pigments that adversely affect the color of the oil. However, removal of these color pigments requires substantially less silica and clays than is required to remove non-hydratable phospholipids. For example, silica usage may be as low as 0.05%, and bleaching clay may be only about 0.5% of the weight of the oil.
Finally, chemicals that add flavors to the oil are removed by a process known as xe2x80x9cdeodorizationxe2x80x9d, which is essentially a form of physical distilling, in which the oil is subjected to high temperatures under a vacuum for a short period of time, which is sufficient to remove the flavor-causing components, but insufficient to break down non-hydratable phospholipids.
Thus, there remains a continuing need for a soybean oil refining process in which free fatty acids could be removed during the deodorization or physical refining step of the refining process, instead of by chemical or caustic refining. In addition, there is a need for a soybean oil extraction process that would permit soybean oil extraction from soybeans, without the need for solvent extraction, such as with hexane, moreover, there is a need for a non-solvent extraction process that produces a lower level or non-hydratable phosphotides.